The present invention relates to a crushing apparatus and a crushing method.
In prior art, it is known to preliminarily coarsely crush materials to be crushed such as cement clinker, cement material, slug, ores and the like by utilizing a vertical roller mill which is excellent in coarse crushing efficiency. The thus coarsely crushed materials are thereafter finely crushed by utilizing a tube mill, for example, which is excellent in fine crushing efficiency.
One of examples of such prior art is disclosed in, for example, the Japanese Patent Laid-Open Publication No. 238349/1986, which is shown in FIG. 10.
FIG. 10 shows a schematic diagram of a crushing apparatus for crushing materials such as cement clinker and referring to FIG. 10, materials to be crushed fed into a vertical roller mill 2 through a material feed port 1 are coarsely crushed in the vertical roller mill 2. The crushed materials are then fed towards a separator 4 through a bucket elevator 3. The coarsely crushed materials classified in the separator 4 in accordance with the sizes of the materials are guided into a tube mill 5 in which the coarsely crushed materials are further crushed finely. The finely crushed materials are returned to the separator 4 through the bucket elevator 3. The finely crushed materials returned to the separator 4 are again classified and then taken out as products through a chute 6. Dust is collected by a dust collector.
Such crushing apparatus as shown in FIG. 10 may be called "one-pass crushing apparatus" because one crushing process is performed by the vertical roller mill 2. In such one-pass crushing apparatus, the materials to be crushed are coarsely crushed in the vertical roller mill 2, and for example, cement clinker having a grain size of 50 mm to 1 mm is crushed to grains having a size of 15 mm to 0.01 mm. The vertical roller mill 2 generally includes a table rotatable around its perpendicular axis and a plurality of rollers arranged on the table in a circumferentially spaced relationship.
The materials are fed through the inlet port 1 on substantially the central portion of the table of the vertical roller mill 2 and then crushed in a gap between the rotating table and the rollers by biting the materials in the gap and then applying pressure to the rollers. For this crushing operation, a perpendicularly downwards-acting large pressing force is applied to the rollers by a pressing means such as hydraulic cylinder means thereby causing large pressing force between the rollers and the table.
Accordingly, in the utilization of the vertical roller mill of the structure described above, large vibration is generated irrespective of the sizes of the materials to be crushed. The generation of such large vibrations constitutes a significant problem for persons who design the crushing apparatus to reduce them to as small as possible.
In a case where such vertical roller mill 2 is utilized as an apparatus for preliminarily crushing materials, it will be initially required for the vertical roller mill to crush materials of relatively large grain sizes and to effectively crush the same in substantially one crushing process. Because of such requirements, it will be necessary for such a vertical roller mill to have a large pressing force or power in comparison with a generally utilized vertical roller mill.
Namely, since the materials fed into the vertical roller mill 2 contain materials, which in percentage, are coarse and have relatively large sizes, it is therefore necessary to effectively crush the materials with a reduced crushing operation time, resulting in the requirement of large pressing force.
As described above, therefore, since the vertical roller mill utilized as the preliminarily crushing means generates large vibrations when compared with a usual vertical roller mill, in the actual operation, it is necessary for the preliminarily crushing type vertical roller mill to be operated with a pressing force of the rollers at a value considerably lower than a desired value for the effective crushing operation, thus causing a significant problem. In addition, from a mechanical point of view, an additional attention has to be paid to obtain a vibration-proof design, thus being not economical.
Furthermore, materials to be newly crushed usually have various sizes, properties such as physical characteristics and the like, and these materials will be continuously fed in the actual crushing operation. Such variations or changes of the sizes and properties of the newly fed materials directly affect a one-pass crushing apparatus of the vertical roller mill type shown in FIG. 10.
Thus, in a case where materials having different sizes are fed, the vibration level or degree will change, and in association therewith, the driving capacity of the vertical roller mill has to be changed, thus also providing a troublesome problem.
In a case where materials having different properties are fed and materials which are hardly crushed are fed, the crushing capacity of a crushing plant is degraded, so that an amount of the material to be supplied to the vertical roller mill is reduced, and hence, the thickness of the materials to be taken into the gap between the rollers and the table of the vertical roller mill is also reduced. As a result, the vibration level will increase and the crushing capacity of the crushing plant will be further degraded. On the contrary, when materials relatively easily crushed are fed, the crushing capacity of the crushing plant can be increased, so that the material thickness between the rollers and the table is also increased, resulting in a decrease of the vibration level. However, in this case, since the material thickness increases, a pressure receiving area of the materials for receiving the roller pressure is increased, so that the press crushing force to be applied to a unit area will be substantially reduced, resulting in the lowering of the actual crushing efficiency.
As described above, in the so-called one-pass crushing apparatus, the grain sizes and the properties, of the materials to be crushed directly affect operation so that the crushing apparatus cannot always be operated under optimum constant and stable crushing conditions.
FIG. 11 represents another example of the prior art such as disclosed in the Japanese Patent Laid-Open Publication No. 116751/1988. FIG. 11 shows a schematic diagram of a crushing apparatus, and referring to FIG. 11, materials fed through a material feed inlet port 8 into a vertical roller mill 9 are once preliminarily or primarily crushed therein and the crushed materials are conveyed to a screening device 11 through a bucket elevator 10. The preliminarily crushed materials are screened by a screening surface 12 of the screening device 11, and the coarse materials in the materials fed from the vertical roller mill 9 each having a grain size larger than a predetermined size more than 2.5 mm, for example, are separated in the screening device 11 and then returned to the vertical roller mill 8 through a chute 13 for re-crushing the coarse materials.
On the contrary, relatively finely crushed materials separated from the coarse materials described above are conveyed to a tube mill 15 through a chute 14 for secondary fine crushing operation. The materials finely crushed by the tube mill 15 are classified by a separator in accordance with their grain sizes, and the materials each having a grain size larger than a predetermined value are again returned to the tube mill 15 for re-crushing the same. The fine materials not returned to the tube mill 15 are taken out therefrom as products through a chute 17. A crushing apparatus of such a type may be called a screening re-circulation type of crushing apparatus because of its nature.
The latter mentioned prior art crushing apparatus aims, in comparison with the former mentioned prior art crushing apparatus, to improve the crushing efficiency of the tube mill, that is, to feed the crushed materials of further small grain sizes to the tube mill by screening the coarse materials, each having a grain size more than the predetermined size such as 2.5 mm, once crushed by the vertical roller mill and returned again to the vertical roller mill for the re-crushing thereof.
However, in the latter mentioned screening re-circulation type crushing apparatus, since the materials once crushed but each having grain size of more than a predetermined value such as more than 2.5 mm are returned to the vertical roller mill 9, the grain sizes of the materials in the vertical roller mill 9 are not substantially changed even after the preliminarily or primarily crushing operation therein. In other words, even in the screening re-circulation type crushing apparatus, the problem of causing large vibrations during the crushing operation is not solved, as is caused in the one-pass type crushing apparatus.
In addition, in the screening re-circulation type crushing apparatus, the influences caused by the change of property of newly fed materials to be crushed in the vertical roller mill are given more largely to the vertical roller mill than the case of the one-pass type crushing apparatus. These influences will be also imparted proportionally to the entire operation of a plant, thus increasing the possibility of instable crushing operation thereof.
Namely, in this crushing apparatus, the materials once crushed by the vertical roller mill are separated into coarse and relatively fine ones by the screening device, the coarse ones then being returned to the vertical roller mill and the fine ones being fed to the tube mill for the secondary finely crushing operation.
Accordingly, amounts of the coarse and fine materials after the screening operation are decided by the grain sizes of the materials crushed by the vertical roller mill. As a matter of nature, the grain sizes of the materials to be fed into the vertical roller mill are not usually constant, and in addition to this fact, the returned coarse materials to the vertical roller mill varying in their amount are continuously added to and mixed with the materials newly fed into the vertical roller mill.
Accordingly, the total amount in the vertical roller mill always changes and this fact magnifies the change of the property of the materials to be crushed in the vertical roller mill as well as the changes of their grain sizes. This will be applied even to a case where the feeding amount of the materials to be newly fed into the vertical roller mill is relatively constant.
Consequently, the magnitude of vibrations of the vertical roller mill always change, as well as the amount of electric power consumption for the vertical roller mill.
Furthermore, since the amount of the secondary separated fine materials to be fed to the tube mill 15 always varies in size, the tube mill 15 cannot be constantly stably operated. Namely, in the screening re-circulation type crushing apparatus of the conventional type described above, the circulation amount of the materials to the vertical roller mill 9 and the supply amount thereof to the tube mill 15 are not optimumly controlled in accordance with the change of properties of materials to be newly fed.
Furthermore, in a crushing plant such as for crushing cement clinker, the crushing capacity is usually of 100 ton/hour to 150 ton/hour, and when materials of such an amount are subjected to screening treatment, materials of about 130 to 200 ton including the circulation amount must be treated per one hour, thus requiring the screening device to have a considerably large size and treating capacity. It is also difficult to use the screening net means for a long time, as it is uneconomical and troublesome to maintain.